Melt butter in a small saucepan over medium heat.  Whisk in flour and cook until bubbly.  Slowly whisk in hot milk until no more lumps remain. Cook until sauce thickens, then whisk in mustard and a pinch of salt.

On a lightly floured surface, roll puff pastry into a roughly 12x12" square. Trim to neaten up the edges, then divide into 4 equal pieces and arrange on a parchment-lined baking sheet; transfer to freezer to chill for 10 minutes.

Meanwhile, preheat grill over medium heat.  Place the puff pastry squares on grates and close the lid.  After 30 seconds, open and rearrange squares on the grates (move the front ones to the back, back ones to front) for even cooking. Close lid, then after 30 more seconds, open it, flip the pastry and reduce flame to low. Top with creamy mustard sauce, cheese and ham.  Close lid and cook until cheese melts, 3-5 minutes. Peek every now and then to make sure the bottoms aren't getting too brown, if they are, turn off the gas and just keep the lid closed until cheese finishes melting. Sprinkle with chives and serve hot.